You can actually get a PC-Background-Picture. For your PKMN-PC. Y'know.
If you wanted a cretain combination of background and a symbol-theme, with each their own color, you had to put in a cretain code. That code also changed with your trainer ID.

As an example:
If you wanted this box-background right here.
AND if your Trainer-ID would be '28535', you'd have to put in this code:
qKnBBBWBVBBBVhs
And that code would get you THSI box-background, ONLY if your Trainer-ID was 28535.